Experimental　study　was　conducted　to　investigate　antifouling　effect　of　ultrasound　to　convective　heat　transfer　process　with　cooling　water　as　working　fluid　in　different　hardness　and　temperature.　An　ultrasonic　treatment　device　was　preinstalled　before　a　double-tube　heat　exchanger.　The　ultrasound　was　applied　and　controlled　in　the　range　of　28-40　kHz　in　frequency　and　25-50　W　in　power.　A　remarkable　effect　of　antifouling　was　observed　from　experimental　results　of　heat　transfer　process　with　ultrasonic　treatment.　There　was　lower　fouling　resistance　which　means　better　antifouling　effect　in　lower　frequency　of　20　kHz　than　that　in　higher　frequency　of　40　kHz.　But　a　contrary　result　was　observed　for　ultrasonic　power,　i.e.　a　higher　antifouling　rate　was　reached　with　the　high　power　of　50　W　ultrasound.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.